1. Field of the Invention
The present invention relates to a trench-gate type semiconductor device and a fabrication method thereof, and more particularly to a transistor, which has open ends of trenches made of materials having a different oxidation rate, and a fabrication method thereof.
2. Description of the Related Art
A high integrated circuit, which has semiconductor devices such as transistors highly integrated by a micromachining technology, is demanded to be more densely integrated and have high driving performance.
In recent years, a semiconductor device having trenches (grooves) is particularly attracting attention. By forming gates so to have a trench configuration, an area occupied by the gates on a substrate can be decreased, resulting in achieving a high integrated circuit which has a high current value and high performance.
However, such a trench configuration has a drawback that a gate oxidation film on the open ends of the trenches is thinner than the other portion of the gate oxidation film. This is because the film grows differently in parts depending on its orientation during the thermal oxidation of the film. In a process for manufacturing a transistor, the oxidation of the gates requires a temperature of about 1000.degree. C. Therefore, the gate oxidization film of the open ends becomes thin. When the gate oxidation film on the open ends of the trenches is thinner than the gate oxidization film on the other portion, it may be broken due to a high electric field generated on the open ends.
Where the oxidation is made at a temperature higher than 1000.degree. C., the gate oxidation film is uniformly formed including the open ends of the trenches. But, the oxidation at a temperature higher than 1000.degree. C. involves a disadvantage that impurities are excessively dispersed in an impurity dispersion layer.
A semiconductor device having a conventional trench configuration and a fabrication method thereof will be described with reference to FIG. 9 through FIG. 13.
FIG. 9 is a plan diagram showing a conventional semiconductor device comprising a gate electrode extension section 11, a source 3 and trenches 7. FIG. 10 is a sectional diagram taken along line A-A' of FIG. 9, and FIG. 11 is a sectional diagram taken along line B-B' of FIG. 1. They show a semiconductor substrate (drain) 1, a base 2, a gate film 8 and a gate electrode 10 as well as the gate electrode extension section 11 and the source 3.
FIGS. 12A to 12D show a process for fabricating a conventional trench-gate type semiconductor device. FIG. 12A is a diagram showing that the base 2, the source 3 and a thermal oxidation film 4 are formed on the silicon substrate (drain) 1. FIG. 12B shows that a trench masking material 6 is formed on the thermal oxidation film 4 to etch trenches. FIG. 12C shows a state that the trenches 7 are formed in the semiconductor substrate by etching, and the thermal oxidation film 4 and the masking material 6 are removed. FIG. 12D shows that a gate film 8 is formed to cover the trenches 7, the base 2 and the source 3, and the gate electrode 10 is formed on the gate film.
FIG. 13 is an enlarged sectional diagram showing a trench configuration of a conventional semiconductor device. It is seen from FIG. 13 that the gate film is thin as if it was notched at both ends of the open surface of the trench. An electric field tends to concentrate on the notched portions 12 where the gate film is thin, resulting in degrading a withstand pressure of the gate.
In order to remedy the above-described disadvantage involved in the trench configuration, a variety of attempts are being made.
For example, Japanese Patent Laid-Open Application No. Hei 7-249768 discloses a semiconductor device having the thickened upper end corners of trench side walls by oxidizing increasingly. Japanese Patent Laid-Open Application No. Hei 5-47919 discloses a semiconductor device which is formed to have round edges on trenches where elements are formed. Japanese Patent Laid-Open Application No. Hei 7-326738 discloses a configuration that the upper corners of trenches are not covered with the gate electrode. Japanese Patent Laid-Open Application No. Hei 2-113548 discloses a configuration that stepped sections are formed just below the gate electrode, and an oxidation film is formed on side walls of the stepped sections.
The manufacturing processes in connection with such publications, however, were complex and poor in fabricating the notched parts of the gate film with a satisfactory thickness.
Thus, there are demands for a semiconductor device having a configuration that a notched part is not caused in the gate film on the open ends of the trenches.
In view of the circumstances described above, an object of the invention is to provide a semiconductor device which has a film with a uniform thickness even on the open ends of trenches without causing a notched part by using materials having a different oxidation rate, and a fabrication method thereof.
Another object of the invention is to provide a semiconductor device having trenches with more reliable open ends by filling impurities into a material having a fast oxidation rate among other material having a different oxidation rate in order to further differentiate its oxidation rate from the other materials and a fabrication method thereof.